OBJECTIVES: We compared the male sexual partners of teen girls of age 15 to 19 years, currently infected with a sexually transmitted infection (STI) versus the male partners of adult women of age 20 to 41 years, with an STI to determine risk factors in these high-risk sexual dyads related to the male partner. STUDY DESIGN: Interview of 514 men who were partnered with 152 teen girls and 362 adult women, enrolled in Project Sexual Awareness for Everyone, a randomized controlled trial of behavioral intervention to reduce recurrent STIs. RESULTS: Compared to the male partners of adult women, male partners of teen girls were significantly more likely (P < 0.05) to be infected with any STI at intake. Men partnered with teens were younger and had significantly more sexual partners per year sexually active, shorter relationship length, and shorter length of monogamy with the index girls. They were more likely to report that it was "really important" for the teen to have their baby (P = 0.04) and were slightly more likely to be the father of her children (P = 0.17). Young age independently predicted STI infection in men. CONCLUSIONS: Although all women had an STI at intake, important differences were noted among the male partners of teens versus adults. Clinicians with similar populations may use this data to understand the characteristics of male partners of teens with STIs, in order to more effectively counsel adult and teen women on partner notification, treatment and STI prevention.